Aging hearts exhibit increased cardiac myocyte apoptosis in basal conditions and in response to pathologic insults. Reactive oxygen species (ROS) play an essential role in mediating cardiac myocyte apoptosis. Thioredoxin (TRX) is a multifunctional and ubiquitously expressed 12kD protein. TRX, thioredoxin reductase and NADPH, collectively called the TRX system, operate as a powerful NADPH-dependent protein-disulfide reductase system. TRX is unique among anti-oxidants because TRX promotes cell survival through interaction with regulators of apoptosis/cell survival. However, the importance of TRX in regulation of ROS and cell survival is poorly characterized in cardiac myocytes. Our preliminary results indicate that cardiac myocyte apoptosis and the cellular content of TRX increase in aging monkey hearts. Expression of TRX is also upregulated by pathologic insults in the mouse heart. Transgenic mice with cardiac specific overexpression of dominant negative TRX exhibit increased oxidative stress in the heart and develop cardiac hypertrophy and apoptosis. These results suggest that TRX is upregulated in response to aging and pathologic insults and plays an important role in mediating cardioprotective effects in the aging heart. We hypothesized that: 1) TRX is upregulated in aging hearts in order to counteract age dependent increases in ROS. 2) Upregulation of TRX in response to pathologic insults is blunted in aging hearts, which causes increased susceptibility to pathologic insults in aging hearts. 3) Overexpression of TRX in its reduced form mediates cardioprotective effects, while TRX in its oxidized form mediates increased cell death and facilitates the histological changes seen in aged animals. In order to test our hypothesis, we will use cultured cardiac myocytes and transgenic mice harboring wild type or dominant negative TRX. We will also use the conscious monkey ischemia/reperfusion model in conjunction with adenovirus-mediated gene transfer in order to manipulate expression of TRX in the heart. Our study will lead to a better understanding of the mechanisms of cell death in aging hearts. Elucidation of anti-oxidative and cell survival effects regulated by TRX will lead to development of novel and specific strategies to reduce ROS and prevent cardiac myocyte death in aging hearts.